NarutoBase.net - Naruto Manga Chapter 467 - Page 18, Madara said he has the power of the bijuu (tailed beasts) he collected thus far. It seems risky for Madara to use the tailed beasts since they could be captured and hidden away. We've already seen evidence of this happening at the beginning of Naruto 494 (NarutoBase.net - Naruto Manga Chapter 494 - Page 1).
The 8 tails is stronger than all the previous 7 tailed beasts that were captured and the 3rd Raikage and his elite squadron were enough to capture the 8 tails.
If Madara sends the tailed beasts out and some of them are captured and hidden away that would really screw up hs plans.